The present invention relates to a nuclear magnetic resonance imaging (hereinafter, referred to as "MRI") method and apparatus in which density distribution or relaxation time distribution is imaged by measuring nuclear magnetic resonance (hereinafter, referred to as "NMR") signals emitted from hydrogen or phosphorous in an object to be imaged.
In an MRI apparatus widely used in the field of clinic in the present years, obtained is geometrical information utilizing protons which are main constitutional elements of an object to be imaged. MR images widely used in the field of clinic in the present years are a two-dimensional cross-sectional, a two-dimensional sagittal-sectional and a two-dimensional coronary-sectional images of the head, the cervix, the spinal column, joints or the like. In a case of obtaining these images, often used is multi-slice imaging where a plurality of laminated cross-sections are sequentially imaged.
There are various imaging methods of an MRI apparatus which are characterized by the sequences of magnetic field pulses applied. As the typical imaging pulse sequences, FIG. 1 shows the pulse sequence in a spin echo (hereinafter, referred to as "SE") method, FIG. 2 showing the pulse sequence in a gradient echo (hereinafter, referred to as "GrE") method. In these pulse sequences, gradient magnetic field pulses G.sub.Z, G.sub.Y, G.sub.X having different directions perpendicular to each other are used, respectively. These gradient magnetic field pulses are called a slice selecting gradient magnetic field G.sub.Z 200 to determine a slice, a phase encoding gradient magnetic field pulse G.sub.Y 300 to encode spacial information in the y-direction into the phase of signals, and a read-out gradient magnetic field pulse or frequency encoding gradient magnetic field pulse G.sub.X 400, respectively.
In the SE method shown in FIG. 1, a 90.degree. radio frequency pulse 101 is firstly irradiated while a slice selecting gradient magnetic field G.sub.Z 201 is being applied, and then a phase encoding gradient magnetic field pulse G.sub.Y is applied. Next, a 180.degree. radio frequency pulse 102 is irradiated, and finally a signal is read out while a frequency encoding (read-out) gradient magnetic field pulse 401 is being applied as shown in the figure. The signal is obtained as an echo signal 501.
On the other hand, in the GrE method shown in FIG. 2, it is different from the pulse sequence in FIG. 1. The pulse sequence is the same as that in FIG. 1 except that an echo signal 501 is obtained by reversing a gradient magnetic field pulse G.sub.X 402 instead of applying the 180.degree. radio frequency magnetic field pulse 102.
In order to obtain image data comprising 256.times.256 image elements through such a pulse sequence, the pulse sequence shown in FIG. 1 is basically repeated 256 times by setting the number of data points for reading out the echo signal 501 while the intensity of the phase encoding gradient magnetic field pulse G.sub.Y 301 is being varied step by step. When a sheet of image is detected, the signals are generally detected under the same thickness of slice. The reason is because it is mathematically ideal that all items of the data used in forming an image have the same thickness of slice. The typical thicknesses of slice are 2 mm, 7 mm, 5 mm, 10 mm and so on.
FIG. 3 is a schematic view showing signals obtained in such a conventional imaging pulse sequence. All the slice thicknesses are constant throughout the region 703 of phase encoding. The slicing thickness is generally determined by the amplitude of the slice selecting gradient magnetic field pulse G.sub.Z 201 applied concurrently with the radio frequency pulse 101 and the width of the radio frequency pulse 101. In other words, (1) the slice selecting gradient magnetic field pulse G.sub.Z 201 is applied in a pulse to temporarily produce a spatial-dependent magnetic field for spatially varying the resonance frequency of an object to be imaged, and then (2) a radio frequency pulse having only a certain frequency band, for typical example, 90.degree. radio frequency pulse having a sinc-function-shape, is applied to selectively excite the atomic nuclei existing inside a limited space (slice) among the atomic nuclei constructing the object to be imaged. Therefore, the amplitude of the slice selecting magnetic field pulse G.sub.Z 201 is constant while the signal detection described above is being repeated 256 times.
Although, in the conventional imaging method, an image is reconstructed using the data obtained from the same slice thickness as described above, there is no need in the actual clinical application to satisfy the condition. That is, the spatial frequency components in the internal construction of human body are not homogeneous. In addition, there is the most important frequency for diagnosis point of view depending on the subject organ or part. For example, in order to image a fine structure such as a blood vessel, the spatial frequency of 0.25 to 0.5 1 p/mm is important, and the spatial frequency less than the value above is not important for imaging the edge of the fine structure. Furthermore, such a low radio frequency component is three-dimensional isotropic in most cases.
However, since the conventional method uses the slice selecting gradient magnetic field pulse having a constant amplitude independent of the importance of spatial frequency, there have been some cases where a necessary S/N ratio cannot obtained or a high spatial resolution cannot be obtained depending on the organ or the part.
In order to obtain multi-slice images through the conventional imaging method of this type, it is required to repeat the 256 times of signal acquisition sequences as many times as the number of slices required with varying the amplitude of the slice selecting gradient magnetic field pulse 201. For example, when 10 slices are required, the repeating number of signal acquisition sequences totals 256.times.10=2560 times. That is, the signal acquisition is repeated as many times as the number of slices by varying the intensity of the slice selecting gradient magnetic field G.sub.Z.
In the SE method shown in FIG. 1, it is required to provide a waiting period T.sub.r from a first 90.degree. radio frequency pulse 101 to the following 90.degree. radio frequency pulse 101 (not shown) long enough to wait for the recovery of longitudinal magnetization of spins. Since the value of the waiting period is long, from 300 ms to 2 s, there has been developed a technique where the signal in another slice is acquired during this waiting period. In this case, the time required for measuring is not always 2560 times of a signal acquisition sequence as described above.
However, the number of slices measurable during the waiting period T.sub.r depends on the waiting period T.sub.r and an echo time T.sub.e which is a parameter dominating image quality, and is usually 3 to 10 slices. Therefore, in a case of imaging during a short waiting period T.sub.r or imaging a lot of slices as many as 20 slices, the measuring time increases as a result.
On the other hand, in the GrE method in FIG. 2, the main method is a fast GrE method where signal is continuously acquired without waiting the recovery of the longitudinal magnetization. In this case, in order to obtain 10 multi-slice images, the number of repeating times becomes 2560 times and the measuring time becomes 2560 times as the calculation.
Since the multi-slice images are important from a clinical point of view because of containing a lot of diagnostic information, it is strongly required to improve the speed of measurement because of a lot of repeating times and a long imaging time as described above. Further, as a future important application of the multi-slice imaging, there is the measurement of brain functions. In this measurement, light or sound is used for stimulating a human body and the reaction of the brain against the stimulation is observed as an MRI image. Since the response time of brain activation is approximately 10 seconds or less, it is required to detect a three-dimensional image of brain with a speed of this order.
In the past, in order to improving the imaging speed, efforts have been focused in shortening the time to obtain the image of one slice. Although some of the methods are succeed in practical uses as a fast GrE method, fast SE method, half-scanning method and so on, change in or degradation of image quality to a some extent cannot be avoided as the imaging speed increases.